Sensitometry is the measurement of the sensitivity of photographic materials. A sensitometer is an instrument which gives the photographic material (usually a film) an accurately controlled light exposure. The instrument usually consists of a light source with or without filters, a timing mechanism, and a step wedge of graduated transparency which modulates the intensity of a light exposure in blocks of varying density. The range is from clear to black, usually in incremental steps of 0.3 or 0.15 density. The instrument also includes a means for holding the film during the exposure. "Density" is expressed as the logarithm of the ratio of the incident light on the film to the light transmitted by the film.
Accurately controlled test exposures of film samples are useful in monitoring the sensitivity of the film and also in the processing variables involved in the development of the film. The latter variables include the time of development, temperature, agitation, and the concentration or activity of the solutions.
It is particularly important to monitor the film characteristics and processing variables associated with X-ray films. These films frequently involve film-intensifying screen combinations, which are essentially a sandwich of film pressed between one or generally two X-ray-activated phosphor screens. The screens are provided to reduce the total amount of radiation needed to expose the film. As an X-ray beam hits the first screen, the screen emits a light of a particular color that will expose the film. The X-ray beam then passes through the film, adding to the exposure, and then through the second screen to generate an added quantity of light to the exposure. The generation of the light at the screens thus adds to the primary exposure of the film by the X-ray beam itself, and reduces the necessary beam intensity. It is particularly significant that different types of screens emit different colors. Ideally, the sensitometric exposure should simulate the color exposure of the particular screen used in the film being tested, since the sensitometer will not activate the phosphor screens with X-rays.
Presently available sensitometers use filters positioned between a standard light source and the test film, or a variety of colored lamps to affect the desired color change in the exposing light to adapt the test to the particular film sample. The mechanical structure required for these arrangements, together with the time and inconvenience required to change filters or lamps, establishes a need for a simpler approach to the problem of changing the color of the exposing light.
A variety of light sources have been used in sensitometers, among these being electroluminescent panels. These panels have a construction generally similar to that of a capacitor, in that the space between conductive plates is occupied by a dielectric matrix in which phosphor crystals are embedded. The application of alternating current within particular frequency bands will activate the phosphor crystals, and emit light of certain characteristics. This type of lamp has many advantages over other light sources for sensitometry, and has been used for this purpose. It has uniform brightness over a flat area, is dependable, and is inherently thin and vibration resistant. The light intensity can be fully controlled over a considerable range by control of the amplitude of the excitation voltage.